Resin embedded rock bolts are used in many mining and civil engineering applications, in particular to stabilize rock strata during tunnelling or extractive operations. To install a rock bolt in a mine wall a blind hole is firstly drilled in the wall and a chemical or resin mixture inserted into the hole. The resin is normally contained within a plastic “sausage” with an interior dividing wall which maintains separate the two components which when mixed together cause the resin to set. The bolt is then inserted, and is preferably rotated during insertion, to mix the resin. Naturally, insertion of the bolt pushes the resin “sausage(s)” or cartridge(s) to the far end of the hole, whilst the rotation of the bolt shreds the sheet material from which the sausage or cartridge is fabricated. To enhance mixing, the bolt may be provided with suitable thread form. In the event the diameter of the hole is greater than desired, a mixing device, such as a wire spirally wrapped along the length of the bolt may be provided. Alternatively, a paddle section may be provided by stamping the bolt so as to create lateral protrusions. U.S. Pat. No. 5,054,146 (Wiesenfeld) is illustrative of this stamping art, the protrusions 3 being formed by pins 14 mounted on a pair of rollers 8.
Once rotation of the bolt starts, the resin seals the hole trapping the air around the cartridge in the hole. This air must be mixed into the resin in such a way to evenly distribute its volume. Lateral protrusions of large areas on the bolt tend to cavitate the resin cartridge mixing by drawing the air around the bolt and pushing the high viscosity resin to the outside diameter of the hole. This effectively separates the resin and creates voids. This leads to increased bond failure. The resin setting process is generally thermo reactive so thermal expansion of the air is possible creating larger volume voids or bubbles in the resin.
A disadvantage associated with forming the paddle section by stamping is that substantive compressive load needs to be applied to the bolt. The resultant stresses plastically deform the bolt axially as well as sidewardly of the bolt. Thus, as disclosed in U.S. Pat. No. 5,054,146, the bar requires straightening after formation of the protrusions 3. The sideward displacement of material in the bolt is minimal and control of the shape of the paddle section is difficult to achieve.
It will be appreciated that this mining and tunnelling art is very different from conventional masonry fasteners which are intended to be fixed by a friction fit between the fastener and the interior of the hole into which it is inserted. U.S. Pat. No. 2,006,813 (Norwood) is representative of this friction fit art. It will be seen that the device of Norwood is driven into a hole having a diameter slightly less than the maximum transverse extent of the fastener's shank. The fastener's lateral protrusions are compressed forming a tight friction fit with the hole. It is impossible to rotate the Norwood fastener after insertion and no resin is used in the friction fit art.